Antimicrobial peptides are produced by a wide variety of organisms including bacteria, insects, and mammals (Hancock (1998) Expert Opin. Investig. Drugs 7:167-74; Jack & Jung (2000) Curr. Opin. Chem. Biol. 4:310-7; Toke (2005) Biopolymers 80:717-735). Due to the rapid spread of multiple-drug resistant bacterial strains, antimicrobial peptides are currently being investigated as a potential new source of antibiotics to treat infections. Antimicrobial peptides have a high degree of structural and chemical diversity, exhibit rapid bactericidal action, and typically display a broad spectrum of activity. The lantibiotic group of bacterial-derived antimicrobial peptides possesses high antibacterial activity against Gram positive bacteria including drug resistant strains (Delves-Broughton, et al. (1996) Antonie vanLeeuwenhoek 69:193-202; Kruszewska, et al. (2004) J. Antimicrob. Chemother. 54:648-53; Brumfitt, et al. (2002) J. Antimicrob. Chemother. 50:731-4; Galvin, et al. (1999) Lett. Appl. Microbiol. 28:355-8; Goldstein, et al. (1998) J. Antimicrob. Chemother. 42:277-8; Cotter, et al. (2005) Nat. Rev. Microbiol. 3:777-88). Over 45 members have been identified in the lantibiotic family (Chatterjee, et al. (2005) Chem. Rev. 105:633-84). The most studied lantibiotic, nisin, is produced by Lactococcus lactis and has been used world-wide in food preservation for over 40 years (Delves-Broughton, et al. (1996) supra; Hurst (1981) Adv. Appl. Microbiol. 27:85-123; Rayman, et al. (1981) Appl. Environ. Microbiol. 41:375-80). Lantibiotics share the presence of lanthionine (Lan) and/or methyllanthionine (MeLan) residues, and also typically the unsaturated amino acids dehydroalanine (Dha) and dehydrobutyrine (Dhb). These structural motifs are the basis for their biological activity as well as their family name (Schnell, et al. (1988) Nature 333:276-278).
Lantibiotics are ribosomally synthesized as precursor peptides (prepeptides) that are subjected to post-translational modifications to produce the active, mature compounds (Cotter, et al. (2005) Nat. Rev. Microbiol. 3:777-88; Chatterjee, et al. (2005) Chem. Rev. 105:633-84). The prepeptide contains an amino-terminal leader sequence that does not undergo post-translational modification. The role of this leader sequence appears to be required for modification of the structural region and must be removed by proteolysis in the final step to produce the mature lantibiotic (Schnell, et al. (1988) Nature 333:276-278; van der Meer, et al. (1994) J. Biol. Chem. 269:3555-62; Xie, et al. (2004) Science 303:679-81; Li, et al. (2006) Science 5766:1464-7). The dehydro amino acids (Dha and Dhb) found in lantibiotics are introduced via the dehydration of serine and threonine residues located in the carboxy-terminal structural region of the prepeptide. Lanthionine (Lan) and methyllanthionine (MeLan) rings can then be generated by intramolecular conjugate additions of cysteine residues to these α,β-unsaturated amino acids.
A growing class of two-component lantibiotic systems utilizes two peptides that are each post-translationally modified to an active form and act in synergy to provide antibacterial activity (Garneau, et al. (2002) Biochimie 84:577-92). Dehydration and cyclization of the prepeptides to form lanthionine bridges in these systems is likely performed by bifunctional LanM proteins. In most cases the sequence similarity of the two peptides is rather low (˜25%), and so two different enzymes are thought to be employed for the post-translational modification of each peptide (McAuliffe, et al. (2000) Microbiology 146:2147-54). The exception is cytolysin, a two-component lantibiotic that is processed by a single LanM enzyme (Cox, et al. (2005) Curr. Protein Pept. Sci. 6:77-84). In this case, the sequence homology of the two peptide substrates is much higher at ˜90%. Other post-translational modifications of the peptides in two-component systems can include the conversion of L-Ser to D-Ala (Skaugen, et al. (1994) J. Biol. Chem. 269:27183-27185; Cotter, et al. (2005) Proc. Natl. Acad. Sci. USA 102:18584-9) and formation of amino-terminal α-keto amides from the deamination of dehydro residues (Martin, et al. (2004) Biochemistry 43:3049-3056).
The best-studied two-component lantibiotic, lacticin 3147, is composed of the modified peptides LtnA1 and LtnA2, and is produced by Lactococcus lactis (Ryan, et al. (1999) J. Biol. Chem. 274:37544-50). Since the designation LtnA1 and LtnA2 also refers to the unmodified prepeptides, the designations Ltn1 and Ltn2 are used herein for the mature, active components. The post-translational modification of each prepeptide is believed to be catalyzed by two separate bifunctional enzymes, LtnM1 and LtnM2, based on genetic data in which deletion of either LanM gene results in abrogation of bioactive material (McAuliffe, et al. (2000) supra). To date, in vitro activity of LtnM1 or LtnM2 has not been demonstrated. The Ltn1 and Ltn2 peptides act in synergy in a 1:1 ratio to produce nanomolar antibacterial activity (Morgan, et al. (2005) Antimicrob. Agents Chemother. 49:2606-11). A study on the mode of action of lacticin 3147 demonstrated that Ltn1 binds to the peptidoglycan precursor lipid II (Wiedemann, et al. (Jun. 12, 2006) Mol. Microbiol.), a result that was anticipated because of the structural similarity between Ltn1 and mersacidin, which also disrupts cell wall biosynthesis by binding to lipid II (Brötz, et al. (1998) Mol. Microbiol. 30:317-327). In order for lacticin 3147 to substantially inhibit cell wall biosynthesis and form small pores in the cell membrane, however, Ltn2 is also necessary, leading to a proposed model in which the lipid II:Ltn1 complex recruits Ltn2 to form a high affinity complex (Wiedemann, et al. (Jun. 12, 2006) supra). Structural characterization of the modified peptides has indicated that Ltn1 adopts a globular conformation similar to mersacidin, while Ltn2 has a more elongated structure that is α-helical in nature (Martin, et al. (2004) supra).
The mechanisms governing substrate recognition and specificity in two-component lantibiotic systems that utilize two modification enzymes are of great interest since it is believed that each LanM protein is required to discriminate between the two prepeptides present in the cell. Needed in the art is a method for in vitro reconstitution of a two-component lantibiotic biosynthetic system to provide definitive support for the roles of the proteins involved and demonstrate recognition and specificity. Such a system could be used to develop novel lantibiotics based on designing peptide sequences that can be site-specifically modified to yield new products. Given the synergy observed among two-component lantibiotics, which display similar or higher activity than the best single-component lantibiotic nisin (Morgan, et al. (2005) supra), the engineering of new lantibiotics with therapeutic potential could be realized.